For the generation of X-rays, several approaches are available. The conventional X-ray generation by irradiating a target with an electron beam is typically used in medicine and material science. For microscopy and lithography applications, a laser-based radiation source has been developed, which covers the wavelength range from XUV- and UV-ranges down to X-rays (see e.g. T. Brabec et al. in “Reviews of Modern Physics”, vol. 72, 2000, p. 545). With these laser-based sources, an intense laser beam is focussed onto a target material, like e.g. a Xe jet. Due to a HHG process in the target material Xe, X-rays are emitted as described by e.g. J. Seres et al. in “Nature” (vol. 433, 2005, p. 596). Since the HHG process requires very high intensities (>1013 W/cm2) laser light pulses have to be amplified before being focussed onto the target material.
Techniques for pulse amplification by a coherent addition of light pulses are described e.g. in DE 197 50 320 or by R. J. Jones et al. in “Optics Letters” (vol. 29, 2004, p. 2812).
While laser sources for generating laser light pulses with a high repetition rate (MHz and higher) are available, the required amplification of the laser pulses for concentrating the laser power into a small number of ultra-short pulses per second has limited the repetition rates to a few kHz up to 100 kHz. Therefore, the HHG process can deliver HHG pulses with a relative low repetition frequency only.
To overcome this drawback of a low repetition rate, blue and UV radiation sources have been proposed, which are based on a second harmonic generation (SHG) or third harmonic generation (THG) with an optically non-linear crystal irradiated with ultra-short light pulses (see e.g. V. P. Yanovsky et al. in “Optics Letters”, vol. 19, 1994, p. 1952; M. A. Persaud et al. in “IEEE Journal of Quantum Electronics”, vol. 26, 1990, p. 1253; G. McConnell et al. in “Journal of Physics D: Applied Physics”, vol. 34, 2001, p. 2408; and T.-M. Liu et al. in “Applied Physics Letters”, vol. 86, 2005, p. 061112-1).
With these techniques, the laser light pulses are coupled into a resonant enhancement cavity including the non-linear crystal. Due to the phase-coherent addition of light pulses with the simultaneous SHG or THG process, harmonic light pulses with high repetition rates (up to GHz) can be obtained. However, the conventional generation of short wavelength radiation pulses has a drawback in terms of a limited wavelength range. As an example, M. A. Persaud et al. (see above) have described ps pulses at 243 mm. However, wavelength ranges of X- or XUV-rays have not been reached with the conventional techniques as no crystalline material is available that is transparent below 200 nm.